1. Field of the Invention
This invention relates to optical fiber cables and, in particular, to improved optical fiber cables of the composite buffer type.
2. Description of the Prior Art
As is well known in the art, optical waveguide fibers experience high increases in attenuation when subjected to tensile, bending or torsion strains and are sensitive to crack growth (fatigue) and breakage. Accordingly, extensive efforts have been made to develop protective coverings for fibers which will provide a "buffer" between the fiber and its surroundings.
Three general types of protective coverings have been recognized in the art: 1) loose tube buffers, 2) tight buffers, and 3) composite buffers. See Mahlke, G. and Gossing, P., Fiber Optic Cables, John Wiley and Sons Limited, New York, 1987, pages 96-108. Other specialized constructions have also been developed. See, for example, Saito et al., U.S. Pat. No. 4,770,489, which discusses the use of a grooved spacer core to carry a group of fibers, wherein the grooves are filled with a soft jelly-like material, the spacer is surrounded with a layer of aramid fiber, and the aramid fiber is surrounded by a plastic jacket.
In the basic loose tube construction, one or more fibers are surrounded by a protective tube which is spaced from the fiber(s) by a distance sufficient to allow the fiber(s) to move radially within the tube in response to applied stresses. The spacing is normally at least equal to the radius of the fiber, i.e., the spacing is on the order of at least about 125 microns. To accommodate spacings of this magnitude, the overall diameters of loose tube cables are generally quite large, being on the order of 1,400-2,000 microns for single fiber cables and 2,800-3,000 microns multi-fiber cables.
Typically, the space between the fiber(s) and the loose tube is filled with a water resistant compound, e.g., a grease or a thixotropic gel, so as to protect the fiber(s) from water absorption in case the wall of the protective tube should become broken during installation or use. See, for example, Greveling, U.S. Pat. No. 4,763,982, which discloses a loose tube construction comprising 1) an outer jacket and 2) an inner tube composed of strength members, such as aramid fibers, embedded in a resin carrier, the inner tube being filled with a thixotropic water blocking medium: and Peacock, U.S. Pat. No. 4,822,133, which discloses the use of water blocking grease in a loose tube construction. See also Kinard et al., U.S. Pat. No. 4,844,575, Gartside, III et al., U.S. Pat. No. 4,826,278, Taylor et al., U.S. Pat. No. 4,776,910, Arroyo, U.S. Pat. No. 4,730,894, and Johnson et al., U.S. Pat. No. 4,723,831.
Various constructions for the tube portion of loose tube cables have been disclosed. For example, Siecor Corporation of Hickory, N.C., has sold "Fan-Out Tubing" composed of a PVC outer jacket, a layer of aramid yarn, and a fluoropolymer inner tube. Protective tubes having this composition and an inside diameter (ID) of 400 microns have been used with fibers having an outside diameter of 250 microns to form a loose tube construction. Tubes having the same composition but a larger inside diameter, i.e., 1,000 microns, have been used as an added layer of protection for 900 micron tight buffered fibers. See The Fiber Optic Catalog--1988-1989, page 1.20, 1988.
Other loose tube examples include Ditscheid et al., U.S. Pat. No. 4,659,174, which discloses a construction in which at least one optical fiber is embedded in a bundle of strength fibers, such as aramid fibers, and that combination is surrounded by a protective mantle; and UK Patent Application No. 2,185,828 which discloses the use of embedded aramid fibers as strength members in a loose tube construction.
The second type of buffer--the tight buffer--is used with single fibers. In this construction, the protective tube is applied directly to the fiber so that the fiber and the tube are in contact along substantially their entire length, rather than being spaced apart as in a loose tube construction. This change allows the overall diameter of the buffered fiber to be reduced to approximately 900 microns. The buffered fiber is built up to larger diameters as cable.
Again, a variety of constructions have been proposed for the protective tube. For example, Siecor Corporation has sold "Interconnection Cables" composed of a PVC outer jacket, a layer of stranded aramid yarn, and a layer of a polyester elastomer in direct contact with the fiber. See The Fiber Optic Catalog--1988-1989, pages 1.2 and 1.3, 1988.
Other tight buffer examples include Ueno et al., U.S. Pat. No. 4,778,245, which discloses a construction in which a tension resistant material (polyester fibers in the reference example; polyethylene terephthalate yarns and aramid fiber yarns in the comparative examples) is applied directly to an optical waveguide fiber and the resulting structure coated with a synthetic resin such as polyethylene; Ramsay et al., U.S. Pat. No. 4,756,600, which discloses a construction comprising a urethane acrylate primary coating, a silicone rubber secondary coating, and a nylon tertiary coating; Suzuki, U.S. Pat. No. 4,741,594, which discloses a construction comprising a silicone resin in contact with a fiber and surrounded by strength members which are held in place by a layer of tape which, in turn, is surrounded by a sheath of polyvinyl chloride or a similar material; and Stiles, U.S. Pat. No. 4,365,865, which discloses a construction in which a layer of silicone rubber surrounds the fiber, a layer of fiber-reinforced resin, such as an epoxy resin reinforced with aramid fibers, surrounds the silicone rubber layer, and a layer of polypropylene surrounds the fiber-reinforced layer.
Other multi-layer tight buffer constructions employing fiber-reinforced layers can be found in Fuse et al., U.S. Pat. No. 4,629,286, Yoshihara et al., U.S. Pat. No. 4,645,297, Nakasone et al., U.S. Pat. No. 4,795,234, UK Patent Application No. 2,078,996, and EPO Patent Publication No. 284,667. Other constructions employing a layer of non-embedded strength members can be found in Ohta et al., U.S. Pat. No. 4,779,953, and UK Patent Application No. 2,086,607.
The composite buffer construction, like the tight buffer construction, has been used, prior to the present invention, to protect single fibers. In this construction, the fiber is separated from the tube by a distance of between about 50 and about 100 microns, i.e., the composite buffer construction differs from the tight buffer construction in that the tube and the fiber are substantially mechanically decoupled from one another and differs from the loose tube construction in that the spacing between the fiber and the tube is less than the radius of the fiber. The space between the fiber and the tube is normally filled with a fill compound to provide water protection for the fiber. As a result of the use of a reduced space between the fiber and the tube, composite buffer cables have had an overall diameter similar to that of tight buffer cables, i.e., an outside diameter of about 900 microns.
Although composite buffer constructions have worked successfully in various applications, they have suffered from a number of problems. First, although their 900 micron outside diameters represent an improvement over the loose tube construction, this diameter is still too large for applications in which space is at a premium. For example, an important application for optical waveguide fibers is in the field of cable television. For this application, the fibers are installed in existing electrical plenums which typically have small cross-sectional areas. For large buildings, e.g., apartment buildings, numerous fibers must be carried by each plenum. Accordingly, even an outside diameter of 900 microns is often considered too large for this application.
In addition to the size problem, composite buffer constructions and, in particular, composite buffer constructions having small diameters, have had inferior protective properties. Specifically, prior to the present invention, composite buffer constructions have not included strength members. Such members have not been used because they are difficult to incorporate in a composite buffer construction while maintaining a controlled spacing between the fiber and the protective tube. Also, the inclusion of strength members is known to increase the overall size of the cable which would defeat one of the primary reasons for using the composite buffer construction.
Significantly, as discussed in detail below, the present invention provides methods for including aramid strength members in a composite buffer cable and, at the same time, reducing the cable's outside diameter below 900 microns. That is, in accordance with the invention, both the size problem and the protective property problem of the prior composite buffer constructions have been addressed and solved simultaneously.
In addition to the size and space problems, prior to the present invention, multi-fiber cables having a composite buffer construction have not been produced. That is, the art has not produced a cable in which multiple fibers are 1) surrounded by a protective tube, 2) mechanically decoupled from the tube, and 3) closely spaced to the tube so as to produce a high fiber packing density. As discussed in detail below, the present invention provides such a construction and thus satisfies the long standing need in the art for a cable having a high packing density and superior performance characteristics.